12th House
The twelfth house is the house of the hidden: solitude, the subconscious, closed institutions, and whatever a person shows to no one.
What the House Rules
The twelfth house rules what happens behind the scenes: inner life, solitude, secrets, and, in the classical tradition, hospitals, prisons, and monasteries, that is, places where a person is isolated.
The modern reading is gentler: it is the area of recovery, inner work, and what one has yet to become aware of.
Planets in the House
Neptune in the twelfth house is at home: a strong imagination and a tendency to withdraw into oneself. Saturn means loneliness and fears. The Sun means a person whose true nature is hidden from outsiders. Mars means suppressed aggression that comes out unexpectedly.
Sign on the Cusp and an Empty House
The sign on the cusp describes what a person doesn't show and how they rest.
Popular texts call the twelfth house "the house of enemies and misfortune," which is a crude simplification of the classical meanings. There is no reason to expect trouble because of a planet in this house.